There are many examples of conventional photonic devices which exploit nonlinear optical properties of materials to convert light of a first frequency to light of a second frequency. Common examples exploit the nonlinear optical properties to provide second harmonic generation (SHG) whereby light with a first frequency emitted by a laser source (a “pump” laser) is converted to light with a second frequency which is double the first frequency. This process is commonly referred to as frequency-doubling. The frequency-doubled light is laser-like, meaning it has many features similar to the characteristics of light emitted by lasers such as narrow range of wavelengths, high beam quality and strong linear polarisation. Photonic devices which emit frequency-doubled light are also often referred to as lasers or frequency-doubled lasers. Frequency-doubling is often used to provide lasers with emission wavelengths which are difficult or impossible to achieve using direct lasing.
The efficiency of frequency-doubling can be sensitive to changes in conditions including the temperature of the frequency-doubling nonlinear optical material and the wavelength of input light. This sensitivity often necessitates use of components to actively stabilise the temperature of one or more components in frequency-doubled lasers which are deployed in situations where the ambient temperature or other conditions may vary. One important category of frequency-doubled lasers are those configured to provide emission of deep ultraviolet (UV) light (that is, light with wavelength between ≈200 nm and ≈300 nm). There is substantial demand for compact, high performance and low-cost light sources of deep UV light, and especially for deep UV light lasers or laser-like sources. The demand is high because deep UV light may be used for efficacious chemical-free disinfection of bacteria and viruses and to enable sensors for chemical or biological compounds owing to characteristic fluorescence, absorption or scattering of the deep UV light. There are no laser diodes available at deep UV wavelengths.
There are examples of conventional devices for generating deep UV light using frequency-doubling of the light emitted by laser diodes. Nishimura et al. [Japanese Journal of Applied Physics 42 p5079 (2003)] describes a system to frequency-double an input beam with a wavelength of 418 nm using a bulky and complex optical resonator structure to generate an output wavelength of 209 nm. Tangtrongbenchasil et al. [Japanese Journal of Applied Physics 45 p6315 (2006)] describes a system to frequency-double an input beam with a wavelength of 438 nm using another bulky, complex design with a temperature controller applied directly to the nonlinear optical component (a β-BaB2O4 crystal) to hold the component at a stable temperature to generate 219 nm wavelength. Tangtrongbenchasil et al. [Japanese Journal of Applied Physics 47, p2137 (2008)] describes a system to frequency-double an input beam with a wavelength of 440 nm from a laser diode using another bulky, complex design in which the temperature of the laser diode is stabilised using a thermoelectric cooler (TEC), which nonetheless yields an output of frequency-doubled light (wavelength 220 nm) with very low optical power (≈200 nW). Ruhnke et al. [Optics Letters 40, p2127 (2015)] describes a system to frequency-double an input beam with a wavelength of 445 nm to generate a 222.5 nm wavelength output using another bulky, complex design, including an oven to assure stable temperature of the nonlinear optical frequency-doubling component (a β-BaB2O4 crystal) at 50° C. Other features for frequency-doubled lasers capable of emitting deep ultraviolet light using laser diode pump lasers are disclosed in U.S. Pat. No. 8,743,922B2 (Enescu et al., issued on Jun. 3, 2014) and US20150177593A1 (Smeeton et al., published on Jun. 25, 2015).